The present invention relates to a polyamide resin pellet for a miniature part. More particularly, the present invention relates to a polyamide resin pellet which is excellent in heat resistance and low water absorption and exhibits low appearance discoloration and which is capable of providing a molded item or article having a low dispersion of weight and strength and is especially suitable for use in a molding material for forming miniature parts for, for example, motor vehicles and electrical or electronic equipments, and relates to a molded item or article produced from the polyamide resin pellet.
For example, nylon 6 and nylon 66 are widely known as polyamides. In recent years, the demand for a thermoplastic polymer of high heat resistance has increased, and high-melting-point polyamide 46 (which is a polyamide constituted of 1,4-diaminobutane component and adipic acid component, hereinafter referred to simply as xe2x80x9cPA46xe2x80x9d, having a melting point of 290 to 295xc2x0 C.) and semiaromatic polyamide (having a melting point of 280 to 330xc2x0 C.) have been developed.
These polyamides are widely used as a molding material for, for example, injection molding. The polyamides are often used in motor vehicle parts and parts for electrical or electronic equipments, in particular, connectors.
In recent years, in accordance with the miniaturization of electrical or electronic equipments, molded articles as parts thereof have been miniaturized. Accordingly, with respect to the injection molding machine used for forming such molded articles, one of small screw diameter has become employed.
However, these polyamides have a high melting point, and the plasticization thereof in a screw of injection molding machine is difficult. When polyamide resin pellets of the size conventionally employed for molding materials are used, the reduction of screw diameter has caused such a problem that the plasticization thereof cannot be smoothly effected to thereby invite shear heat buildup at a compression zone of screw, which leads to discoloration of molded articles, and that a measuring time is dispersed to thereby increase the dispersion of weight and strength of molded items or articles.
In these circumstances, there is a strong demand for the development of a molding material which is excellent in heat resistance and low water absorption, exhibits low appearance discoloration and is capable of providing a molded item or article having a low dispersion of weight and strength, which molding material is especially suitable for use in miniature parts for motor vehicles and electrical or electronic equipments.
The present invention has been made with a view toward solving the above problems. Accordingly, it is an object of the present invention to provide a polyamide resin pellet which is excellent in heat resistance and low water absorption, exhibits low appearance discoloration and is capable of providing a molded item or article having a low dispersion of weight and strength, which polyamide resin pellet is especially, useful as a molding material for forming miniature parts for, for example, motor vehicles and electrical or electronic equipments. It is another object of the present invention to provide a molded item or article produced from the polyamide resin pellet.
In an essential aspect of the present invention, there is provided a polyamide resin pellet for a miniature part, comprising a polyamide resin (A) whose melting point as measured by DSC is in the range of 280 to below 330xc2x0 C., which pellets have an average volume of 0.001 to 21 mm3,
the average volume determined by randomly sampling 50 pellets, measuring the weight and specific gravity of each of the pellets and dividing the weight by the specific gravity.
In another aspect of the present invention, there is provided a polyamide resin pellet for a miniature part, comprising a polyamide resin (A) whose melting point as measured by DSC is in the range of 280 to below 330xc2x0 C., which has a circular or elliptic section having an average diameter of 0.5 to 3.0 mm and which has an average length of 1.0 to 3.0 mm,
the average length determined by randomly sampling 50 pellets, measuring the length of each of the pellets by means of calipers and calculating an average of measured lengths,
the average diameter determined by randomly sampling 50 pellets, measuring the maximum diameter (major axis in the case of an ellipse) and minimum diameter (minor axis in the case of an ellipse) of each of the pellet sand averaging them, and calculating an average of averaged diameters with respect to all the pellets.
The polyamide resin (A) for use in the present invention is preferably either PA 46 or a semiaromatic polyamide, the semiaromatic polyamide comprising:
diamine component (a) composed of component units of a linear aliphatic diamine having 4 to 12 carbon atoms and/or component units of a side-chain-having aliphatic diamine having 4 to 12 carbon atoms, and
dicarboxylic acid component (b) composed of 40 to 100 mol % of terephthalic acid component units, 0 to 60 mol % of component units of an aromatic dicarboxylic acid other than terephthalic acid and/or 0 to 60 mol % of component units of an aliphatic dicarboxylic acid having 4 to 20 carbon atoms,
the PA 46 or the semiaromatic polyamide having an intrinsic viscosity of 0.5 to 3.0 dl/g as measured in 96.5% concentrated sulfuric acid at 25xc2x0 C.
In the polyamide resin pellet for a miniature part according to the present invention, it is preferred that the polyamide resin pellet consist of a polyamide resin composition comprising the polyamide resin (A) and at least one compounding additive selected from among an inorganic and/or organic reinforcement (B), an organic and/or inorganic fire retardant additive (C) and an impact modifier (D) constituted of a rubber or an elastomer, which polyamide resin composition contains component (A) in an amount of 20% by weight or more, component (B) in an amount of 60% by weight or less, component (C) in an amount of 40% by weight or less and component (D) in an amount of 40% by weight or less, based on the total weight of components (A) to (D).
In a further aspect of the present invention, there is provided a molded item of polyamide resin produced by molding the above polyamide resin pellet for a miniature part by means of an injection molding machine of 20 mm or less plasticization screw diameter and 3.5 mm or less plasticization region feed depth.
The molded item of polyamide resin according to the present invention preferably has a total weight of 5 g or less.
In still a further aspect of the present invention, there is provided a process for producing a molded item of polyamide resin, comprising molding the above polyamide resin pellet for a miniature part by means of an injection molding machine of 20 mm or less plasticization screw diameter and 3.5 mm or less plasticization region feed depth.
The process of the present invention is suitable for forming a molded item having a total weight of 5 g or less.